1. Field of the Invention
The present invention generally relates to electronic detection devices, and more particularly to an apparatus for tracing and locating an open-ended conductor or a conductor forming a closed circuit, a portion of which may lie above ground and a portion of which may lie below the ground.
2. Description of the Prior Art
The art is replete with techniques and devices for determining the direction to and location of a cable, such as an insulated, current-carrying conductor. Most of these techniques involve the use of one or more inductive sensors, such as a coil or a coil with a high-permeability core, which picks up the electromagnetic signal created by an alternating current in the conductor. See, e.g., U.S. Pat. Nos. 4,119,908; 4,134,061; 4,220,913; 4,295,095; 4,387,340; 4,427,942; 4,438,389; 4,390,836; 4,520,317; 4,542,344; 4,639,674; 4,665,369; 4,672,321; 4,767,237; 4,843,324; and 5,093,622. The general direction to the conductor is indicated when a peak or null signal is detected by the inductor, depending upon its orientation; a tangential orientation gives a peak signal and a normal orientation gives a null signal. A similar technique is used in many devices sold by Minnesota Mining and Manufacturing Company (3M-assignee of the present invention) such as the SCOTCHTRAK TK 3B/6B circuit tracers. Other measurement techniques may also be used under certain circumstances. For example, in U.S. Pat. No. 4,542,334, two electrodes are used to steer a device which buries an undersea cable. The electrodes are located on either side of the cable, and capacitively couple a signal to the cable, which is then detected and is used to provide left/right guidance. The sensing of an alternating current may further be enhanced by certain signal processing methods, such as that disclosed in U.S. Pat. No. 4,942,365.
While the tracing of current-carrying conductors is thus easily accomplished, this is not the case for conductors which have a break, i.e., are open-ended. In such a conductor, since there is no closed electrical path, very little current can be established in the conductor (at least when the conductor has negligible capacitive coupling to the surrounding medium), and so typical current-sensing inductors are relatively useless in the attempted location of such a conductor. It has also not been feasible to use the guidance technique of the '334 patent since that technique presumes that the approximate location of the cable is known, the receiving coupler is placed about the cable, and the cable is located between or very near the source electrodes. When the cable is not so located in the immediate vicinity of the electrodes, the signal coupled to the cable from the electrodes is too weak to be successfully processed to provide a left/right signal.
One device which has partially overcome these problems is described in U.S. Pat. No. 4,686,454. That device uses both inductive sensors and a capacitive sensor; the capacitive sensor is not differential, although it is somewhat directional since it uses an electric field sensing "guarded" electrode. A guarded electrode is simply one in which the sensing element is shielded in certain directions by another metallic plate, which is excited by a potential similar to the electrode potential to eliminate "fringing" flux. The metallic plate acts as a driven shield since a feedback arrangement is used to supply the amplified output signal from the sensing element to the metallic plate. This device, however, suffers from the further requirement that the signal from the capacitive sensor must be added to the signals from the inductive sensors in order to provide reliable conductor location. This limitation is primarily due to the inability of the single capacitive sensor to accurately determine the precise direction associated with the maximum received signal, and thus the signals from the inductive sensors are needed to provide further orientation. Otherwise, reliance on the capacitive sensor signal alone would easily lead to an erroneous determination of the conductor location. Furthermore, the combination of the two signals often creates output results which are confusing. It would, therefore, be desirable and advantageous to devise an instrument which overcomes the foregoing limitations, by providing means for detecting an open-ended conductor which combines the benefits of a directional sensor with a differential sensor. The instrument should further have a magnetic sensor to enable it to trace the conductor when for various reasons the electric field sensors are shielded from the conductor.